The present invention relates to a magnetic brush cleaning device for an image forming apparatus. More particularly, it relates to a magnetic brush cleaning device which controls the density of toner in a magnetic brush material consisting of the toner and a magnetic carrier, by detecting a magnetic permeability or a resistance value that changes in accordance with increase or decrease in the quantity of the toner.
In recent years, image forming apparatuses such as electrophotographic copiers have come into wide use wherein static charges are imparted to the surface of a photoconductor by the corona discharge, the photoconductor surface is exposed to a desired image so as to form the corresponding electrostatic latent image thereon, and toner containing iron powder or the like is attracted to the electrostatic latent image portion and is thereafter transferred onto copying paper. In the image forming apparatus of this type, when the toner electrostatically attracted on the photoconductor surface is transferred to the copying paper, part of the toner remains on the photoconductor surface, so that a good copy is sometimes unattainable. In order to prevent this drawback, various devices for cleaning the photoconductor surface have been proposed.
As the cleaning devices for the photoconductor surface, there have heretofore been, for example, a device using a fur brush, a device using a fibrous web member, a device using a cleaning blade, and a device using a magnetic brush.
The cleaning device using the fur brush has been extensively employed in conventional electrophotographic copiers etc. With such cleaning device, the cleaning effect is good at the initial stage. However, the toner fuses and adheres to the brush with increase in the number of copies taken, and the cleaning property degrades gradually, so that the effective lifetime of the brush is short. Moreover, the device is so constructed as to slidingly rub the surface of the photoconductor intensely, thereby to mechanically remove the remaining toner on the photoconductor surface. This leads to such disadvantages that the photoconductor surface is damaged or is frictionally deteriorated and that the lifetime of the photoconductor is drastically shortened.
Furthermore, the toner swept away by the brush must be, for example, absorbed forcibly by a blower and recovered into a filter box. Therefore, the cleaning device becomes large in size. In turn, the copying machine itself inevitably becomes large in size.
With the cleaning device using the fibrous web member, the residual toner is removed from the photoconductor surface by the use of the web, and hence, it is required to clean the photoconductor surface with a new face at all times. In high-speed copying, accordingly, there is the tendency that the cleaning effect lowers conspicuously. To the end of solving this drawback, large quantities of new webs need to be supplied continuously. Another problem is that the photoconductor surface is damaged due to the sliding contact with the web. Therefore, this device has much room for improvement yet.
The cleaning device using the cleaning blade involves as serious disadvantages, such problems that the blade wears away much and that the photoconductor surface damages severely, particularly when the carrier has mixed in the toner on the photoconductor surface. Besides, these problems are conspicuous in high-speed copiers.
The device using the magnet brush removes the residual toner on the photoconductor surface, principally electrostatically. It is therefore meritorious in that the photoconductor surface is damaged little. On the other hand, however, it has the demerit that when the quantity of the toner in the magnetic brush portion has increased, the cleaning effect lowers suddenly. To the end of suppressing this demerit, the toner needs to be separated and removed from a magnetic brush material whose principal ingredient is the magnetic carrier. By way of example, a toner collecting member is disposed in opposing contact with, or in proximity to, the magnetic brush portion, and the toner in the magnetic brush material is shifted to the toner collecting member, thereby to separate and remove the toner from the magnetic brush material. As an expedient for such toner collection, U.S. Pat. No. 3,580,673 discloses a method in which a voltage opposite in polarity to the toner is applied to the toner collecting member. As other methods, there are considered, for example, a method in which the toner collecting member is formed of a conductive metal piece having an insulating surface and is charged by its friction with the magnetic brush material, and a method in which the toner collecting member is formed of a conductive metal and is caused to generate triboelectricity or an image force owing to its contact with the toner.
However, when the toner is collected by any of the aforementioned methods, the toner in excess of the quantity of cleaning of the residual toner from the photoconductor surface is sometimes collected from within the magnetic brush material.
In this case, the quantity of the toner in the magnetic brush portion decreases extremely, and the carrier coexisting in the magnetic brush material can contrariwise adhere to the photoconductor surface, to spoil the cleaning effect. In other words, the toner density of the magnetic brush portion needs to be controlled and maintained in a predetermined range in order to attain the optimum cleaning effect. Nevertheless, this has hitherto been neglected.